1. Field of the Invention
The present invention relates to an image forming device for correcting errors in registration between monochromatic images that are superimposed to form a multicolor image.
2. Description of Related Art
Conventional image forming devices employing an electrophotographic system, such as laser printers and copy machines, perform image formation in an image forming unit provided with a photosensitive drum. The photosensitive drum includes a charge-generating layer and a charge-transporting layer formed over a base layer. A charge is applied to the photosensitive drum by a corona discharge, and the charged drum is exposed to light, such as laser light or LED (Light Emitting Diode) light, to form an electrostatic latent image on the surface of the drum. After the image is developed with toner or another developer, the developed image is transferred onto a recording medium such as paper. Image formation is completed by a fixing unit or the like that fixes the image onto the recording medium with heat.
Image forming devices employing an electrcphotographic system form color images using colored toner of cyan, magenta, yellow, black, and the like and superimposing monochromatic toner images formed of each color to form a single multicolor image. Normally, this type of image forming device is provided with an image forming unit for each color, but only one fixing unit. A monochromatic toner image formed in each image forming unit is transferred onto a recording medium either directly or via an intermediate transfer member. Registration errors among the superimposed images may occur at this time when there are alignment errors in the relative positions at which the monochromatic images are formed. To avoid this problem, these image forming devices perform a calibration process to correct any registration errors when the image forming device is first turned on and during idle times when a printing operation is not being performed, for example.
In the calibration process, the image forming unit measures the position, density, and the like of each monochromatic image formed on the intermediate transfer member, such as a transfer belt, and the image carrying member, such as a conveying belt. The image forming unit then calibrates the relative positions and adjusts the color densities of each image formed in each image forming unit, based on the results of these measurements. However, the image carrying member on which the monochromatic images are formed may suffer from wear, abrasions, or the like due to extended use and other circumstances. Monochromatic images formed over these abrasions or the like during the calibration process can adversely affect measurements.
In a calibration method described in Japanese patent-application publication (kokai) No. HEI-11-258872 (paragraph 0122), a laser beam is irradiated onto the transfer belt and the amount of light reflected off the transfer belt is measured for one cycle. The density of the monochromatic image (monitor pattern) on the transfer belt is detected at the position having the highest measured value.